EXTRA · Parashas Vayeishev · Passuk 38:8 · not a Torah Temimah citation on this passuk
Then Judah said to Onan, “Join with your brother’s wife and do your dutyaduty Cf. Deut. 25.5. by her as a brother-in-law, and provide offspring for your brother.”
Classical source (EXTRA side-ring): Yevamos 24a / classic yibum.
Sefaria — Yevamos 24a / classic yibum
English: “Come in to your brother’s wife and perform yibum with her” — the mitzvah of yibum takes precedence over the mitzvah of chalitzah.
Derash on the verse (Bavli) — not a formal named middah of the 13. Narrative command grounds the ranking of yibum over chalitzah.
Where yibum is possible, it is preferred to chalitzah; the verse states the duty positively.